1. Field of the Invention
The present invention relates to a multilayer thin film structure for use with soft and hard x-rays, cold and thermal neutrons.
2. Discussion of Related Art
Thin film technology has been widely used to control the reflection and transmission of visible light. However, in the wavelength range of x-rays and neutrons the use of thin films has only recently become practicable. Recent advances in the quality control of Layered Synthetic Microstructures (LSM), or multilayers, allows the use of these structures as x-ray and neutron mirrors.
X-ray diffraction from multilayer mirrors is analogous to x-ray diffraction from perfect crystals where the lattice planes are located in the nodes of the standing wave produced by the superposition of incident and reflected (diffracted) waves for enhanced diffraction efficiency. Multilayer mirrors can be considered as an extension of natural crystals for larger lattice spacings. Therefore, as for crystals, x-ray photons will be reflected from multilayer structures only if the Bragg equation is met:
nxcex=2d sin(xcex8)
where
xcex=wavelength of the incident radiation
d=layer-set spacing of a Bragg structure, or the lattice spacing of a crystal
xcex8=angle of incidence
n=the order of the reflection
The structure of a crystalline solid, a regular three dimensional array of atoms, forms a natural diffraction grating for x-rays. The quantity d in the Bragg equation is the perpendicular distance between the planes of atoms in the crystal. Crystalline structures can now be imitated by thin film multilayers, so x-ray diffraction is no longer limited to structures with naturally occurring d spacings.
In order for a multilayer structure to reflect by imitating a crystal structure, a light element of the lowest possible electron density is layered with a heavy element of the highest possible electron density. The heavy element layer acts like the planes of atoms in a crystal, as a scatterer, while the light element layer behaves like the spacers between the planes of atoms. A further requirement of these two elements is to minimize interdiffusion and interfacial roughness as much as possible.
Multilayers possess advantages over natural crystalline structures because by choosing the d spacing of a multilayer structure, devices may be fabricated for use with any wavelength and incidence angle. Crystals also possess poor mechanical qualities such as resistance to scratching. While some multilayer structures are known to scratch easily, this can be countered by depositing a top coat with better mechanical qualities. For example, a topcoat made of Si can be scratched. However, under proper conditions a topcoat made of Si can have its mechanical properties improved to such an extent that the Si topcoat resists scratching.
One disadvantage of multilayers is that they can undergo radiation enhanced damage when subjected to prolonged exposure of irradiation by hard x-rays. Such radiation enhanced damage can be broadly classified as 1) contamination coating on the surface of the multilayers, 2) structural damage, and 3) damage to the substrate. Any of the above classes of damage can contribute to the degradation of the performance of multilayers over time.
In the case of contamination coating, it involves a thin film caused by the deposition of contaminants from the atmosphere/surrounding environment onto the surface of the multilayers. The contamination coating generally contains elements like C, O2, B, Si, etc. and can exist in the form of an oxide or an inorganic compound or both.
One known way to prevent contamination coating is to place the multilayers in a special environment like vacuum or Helium. However, using such a special environment can be cumbersome to work with and expensive to build and maintain.
Accordingly, it is an object of the present invention to design a multilayer structure that reduces the risk of radiation enhanced damage.
One aspect of the present invention regards an optical element for diffracting x-rays that includes a substrate, a diffraction structure applied to the substrate, the diffraction structure including an exterior surface facing away from the substrate and the diffraction structure capable of diffracting x-rays and a protective layer applied to the exterior surface.
An advantage of the above aspect of the present invention is that it reduces the risk of radiation enhanced damage in a normal environment.
Additional objects and advantages of the invention will become apparent from the following description and the appended claims when considered in conjunction with the accompanying drawings.